Packed columns liquid distribution

Many studies on the flow distribution in random packed beds have been reported in the literature. Mercandelli et al. [8] published a short review of the flow distribution work in random packed trickle bed, which includes the list of various techniques used to determine and quantify the flow distribution. Conventional methods include, for example, collecting liquid at the bottom of the column from different zones while advanced methods include tomographic techniques. Mercandelli et al. [8] used several techniques to quantify liquid distribution in columns of diameters up to 30 cm with three different distributor designs. They used global pressure drop measurements, global residence time distribution (RTD) of the liquid, local heat transfer probes, capacitance tomography and a collector at the bottom of the column. 

The earliest of GC analyses were performed on columns packed with a solid support coated with a nonvolatile liquid phase. Packed columns are not frequently used today as they have been replaced by capillary columns where the hquid phase is immobilized on the internal surface of the capillary. As there are numerous liquid phases available, it is now possible to obtain commercial columns that will separate not only the methyl esters but also the underivatized fatty acids. This advancement obviates the need for derivatization and the associated problems. A typical chromatogram of free fatty acids is displayed in Figure 3. Individual isomers of CLA are now available to aid in the identification of isomers in the chromatogram. Gas chromatography can provide quantitative information on the degree of conjugation, positional, and geometric isomer distribution when suitable standards are available. 

Packed columns must be provided with good initial distribution of liquid across the column cross section and redistribution of liquid at various height intei vals that decrease with increasing column diameter. A wide variety of distributors and redistributors are available. Packed columns should be considered when ... 

Bed limiters commonly are used with metal or plastic tower packings. The primary function of these devices is to prevent expansion of the packed bed, as well as to maintain the bed top surface level. In large diameter columns, the packed bed will not fluidize over the entire surface. Vapor surges fluidize random spots on the top of the bed so that after return to normal operation the bed top surface is quite irregular. Thus the liquid distribution can be effected by such an occurrence. 

Perry et al. [85] point out that packed columns are more dependent on liquid distribution than trayed columns, as can be appreciated by the differences in the way the liquid must flow down the two types of columns. Liquid distribution quality is measured or described as [85] ... 

Hoek [86] proposed a radial spreading coefficient to characterize the liquid distribution. This coefficient is a measure of how quickly a packing can spread a vertical liquid stream radially as the liquid progresses down the column [86]. Radial mixing tends to reduce the effects of... 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

Internal uniform distribution Properly designed devices to distribute and redistribute liquid entering the column is critical to obtain best performance of these types of packings. 

In Reference 14, the authors modified the equations for Hg and Hj as follows (a) eliminate column diameter correction above 24 in. and (b) columns with good liquid distribution probably can allows elimination of the packing height correction. 

Satisfactory operation must be between the upper and lower limits for both liquid and vapor flow rates. At liquid rates below 0.5 GPM per square foot of packing cross-section, liquid distribution is not uniform enough to ensure thorough wetting. At liquid rates between 25 GPM and 70 GPM per square foot of packing, the column is considered liquid-loaded and becomes very sensitive to additional liquid or vapor flow. 

Column Operation To assure intimate contact between the counterflowing interstitial streams, the volume fraction of liquid in the foam should be kept below about 10 percent—and the lower the better. Also, rather uniform bubble sizes are desirable. The foam bubbles will thus pack together as blunted polyhedra rather than as spheres, and the suction in the capillaries (Plateau borders) so formed will promote good liquid distribution and contact. To allow for this desirable deviation from sphericity, S = 6.3/d in the equations for enriching, stripping, and combined column operation [Lemlich, Chem. Eng., 75(27), 95 (1968) 76(6), 5 (1969)]. Diameter d still refers to the sphere. 

Fig. 5.2.3 Identification of rivulets and surface wetting in a packing of 5-mm diameter glass spheres contained within a column of inner diameter 40 mm. The data were acquired in a 3D array with an isotropic voxel resolution of 328 xm x 328 pm x 328 [im. (a) The original image of trickle flow is first binary gated, so that only the liquid distribution within the image is seen (white) gas-filled pixels and pixels containing glass spheres show up as zero intensity (black), (b) The liquid distribu-...

Fig 18. Experimental trickle-bed system A, tube bundle for liquid flow distribution B, flow distribution packing of glass helices C, activated carbon trickle bed 1, mass flow controllers 2, gas or liquid rotameters, 3, reactor (indicating point of gas phase introduction) 4, overflow tank for the liquid phase feed 5, liquid phase hold-up tank 6, absorber pump 7, packed absorption column for saturation of the liquid phase 8, gas-liquid disengager in the liquid phase saturation circuit. (Figure from Haure et ai, 1989, with permission, 1989 American Institute of Chemical Engineers.)... 

The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapour, counter-currently, up the column. In some gas-absorption columns co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. 

Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns. 

In general, the largest size of packing that is suitable for the size of column should be used, up to 50 mm. Small sizes are appreciably more expensive than the larger sizes. Above 50 mm the lower cost per cubic metre does not normally compensate for the lower mass transfer efficiency. Use of too large a size in a small column can cause poor liquid distribution. 

The height of the column for a packed column needs to allow for liquid distribution and redistribution. As the feed enters the column, the liquid above the feed needs to be collected, combined with the feed liquid and distributed across the packing below the feed using troughs,... 

Traditionally, packings have not been used in distillation duties, except for small columns of less than 1 m diameter, and even less for high pressure service. The situation has changed over the last 10-15 years, partly through a better understanding of how packed towers work, particularly with respect to liquid distribution, but also through the development of new proprietary packings. 

Bypassing-Controlled Trayed or packed columns operate with countercurrent flow and can achieve many equilibrium stages in series by good distribution of gas and liquid, and careful control of details. Other devices such as sprays are vulnerable to bypassing and are limited to one equilibrium stage. 

F. Yin, A. Afacan, K. Nandakumar, K.T. Chuang, Liquid holdup distribution in packed columns gamma ray tomography and CFD simulation, Chem. Eng. Process. 41 (5) (2002) 473-483. 

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